I tried that but it messes with the lightstreak animation. Thanks for your reply anyway!
But today I had an epiphany.
Instead of complicating everything with expressions I should have the lightstreak on a comp (“lightstreak”) and then in a new comp I can add “lightstreak” as a layer as well as the video. Then i can do the motion tracking as usual.
I haven’t tried it yet, but i think it should work, Right?